D2D operation is a general term that encompasses, among other things, transmission and/or reception of any type of D2D signals (e.g. physical signals, physical channel etc.) by a D2D communication capable UE and/or by a D2D discover capable UE. D2D operation may therefore also be referred to as D2D transmission, D2D reception, D2D communication, etc.
A D2D UE may be referred to interchangeably as a ProSe capable UE. D2D discovery capable UE is also referred to as UE capable of Prose direct discovery and D2D direct communication UE is also referred to as UE capable ProSe direct communication. The link and/carrier that is used for the ProSe direct communication and ProSe direct discovery between UEs is referred to as sidelink.
A UE typically performs radio measurements on both a serving cell and neighbor cells over some known reference symbols or pilot sequences. The measurements are generally performed on cells on an intra-frequency carrier, inter-frequency carrier(s), and inter-RAT carriers(s) (depending on UE capability, whether it supports a designated RAT). To enable inter-frequency and inter-RAT measurements for a UE requiring measurement gaps, the network typically must configure the gaps.
UE measurements can be done for various purposes, such as mobility, positioning, self-organizing network (SON), minimization of drive tests (MDT), operation and maintenance (O&M), and network planning and optimization. Examples of measurements in LTE are Cell identification (e.g., physical cell identifier [PCI] acquisition), Reference symbol received power (RSRP), Reference symbol received quality (RSRQ), CGI acquisition, Reference signal time difference (RSTD), UE Receive-Transmit (RX-TX) time difference measurement, and Radio link monitoring (RLM), including Out of synchronization (out of sync) detection and In synchronization (in-sync) detection. Channel state information (CSI) measurements performed by the UE are used for scheduling, link adaptation etc. by the network. Examples of CSI measurements or other information in CSI reports include Channel Quality Indicator (CQI), Precoding Matrix Indicator (PMI), Rank Indicator (RI), etc. Such measurements may be performed on reference signals such as Cell-Specific Reference Signal (CRS), CSI Reference Signals (CSI-RS), or Demodulation Reference Signal (DMRS).
A radio network node may also perform various measurements on signals transmitted and/or received by the radio network node. Such measurements may be performed to support different functions such as mobility (e.g. cell selection, handover etc.), positioning a UE, link adaption, scheduling, load balancing, admission control, interference management, and interference mitigation. Examples of such measurements include Signal-to-Noise Ratio (SNR), Signal-to-Interference-and-Noise Ratio (SINR), received interference power (RIP), BLER, propagation delay between UE and itself, transmit carrier power, transmit power of specific signals (e.g. Tx power of reference signals), positioning measurements like TA, and eNodeB Rx-Tx time difference.
To identify an unknown cell (e.g. new neighbor cell) a UE typically acquires the timing of that cell and then a physical cell ID (PCI), where the total number of possible PCIs is typically 504. Subsequently, the UE measures the RSRP and/or RSRQ of the newly identified cell for the UE's own use and/or for use by the network node.
To acquire the timing of the cell and the PCI, the UE typically inspects information in downlink (DL) subframe #0 and/or DL subframe #5, which carry synchronization signals (i.e., a primary synchronization signal [PSS] and a secondary synchronization signal [SSS]). In particular, the UE searches or identifies a cell (i.e. acquires the PCI of the cell) by correlating the received PSS/SSS signals in DL subframe #0 and/or in DL subframe #5 with one or more of the pre-defined PSS/SSS sequences. The use of subframe #0 and/or in DL subframe #5 for PCI acquisition depends upon its implementation.
The UE regularly attempts to identify neighbor cells on at least the serving carrier frequenc(ies). But it may also search cells on non-serving carrier(s) when configured by the network node. To conserve power, the UE may search only in one of the DL subframes #0 and #5. To further save battery power, the UE may search the cell once every 40 ms in non-discontinuous-reception (non-DRX) or in short DRX cycle (e.g. up to 40 ms). In longer DRX cycle the UE typically searches a cell once every DRX cycle. During each search attempt the UE typically stores a snapshot of 5-6 ms and post processes by correlating the stored signals with the known PSS/SSS sequences. In non-DRX the UE is able to identify an intra-frequency cell (including RSRS/RSRQ measurements) within 800 ms (i.e. 20 attempts in total including 15 and 5 samples for cell identification [PCI] acquisition) and RSRP/RSRQ measurement).
Device-to-device (D2D) UEs typically transmit D2D signals or channels in an uplink part of the spectrum. D2D operation by a UE is generally in a half-duplex mode, i.e. the UE can either transmit D2D signals/channels or receive D2D signals/channels. There may also be D2D relay UEs that may relay some signals to other D2D UEs. There is also control information for D2D, some of which is transmitted by D2D UEs and the other is transmitted by eNodeBs (e.g., D2D resource grants for D2D communication transmitted via cellular DL control channels). D2D transmissions may occur on resources which are configured by the network or selected autonomously by the D2D UE.
D2D communication implies transmitting by a D2D transmitter D2D data and D2D communication control information with scheduling assignments (SAs) to assist D2D receivers of the D2D data. D2D data transmissions are according to configured patterns and in principle may be transmitted rather frequently. SAs are transmitted periodically. D2D transmitters that are within the network coverage may request eNodeB resources for their D2D communication transmissions and receive in response D2D resource grants for SA and D2D data. Furthermore, eNodeB may broadcast D2D resource pools for D2D communication.
D2D discovery messages are transmitted in infrequent periodic subframes. ENodeBs may broadcast D2D resource pools for D2D discovery, both for reception and transmission.
D2D communication supports two different modes of D2D operation: mode 1 and mode 2. In mode 1, the location of resources for transmission of a scheduling assignment by the broadcasting UE comes from the eNodeB. The location of the resource(s) for transmission of the D2D data by the broadcasting UE comes from the eNodeB. In mode 2, a resource pool for a scheduling assignment is pre-configured and/or semi-statically allocated. The UE on its own selects the resource for scheduling assignment from the resource pool for scheduling assignment to transmit its scheduling assignment.
PCell interruption of 1 subframe occurs where the UE switches its reception between D2D-to-WAN or WAN-to-D2D. This is because the UE receiver chain needs to be retuned every time the operation is switched from WAN to D2D reception and from D2D to WAN reception. This applies to both D2D discovery and D2D communication capable UEs. It is generally beneficial to partition uplink resources between cellular uplink and D2D operation in a way that avoids or minimizes the risk of switching occurring in a certain subframe(s), e.g., subframe #0 and/or #5, of PCell. These subframes contain information such as PSS/SSS for doing cell search, carrying out cell measurements and they also contain MIB/SIB1 information for SI reading procedures. In addition to interruption due to switching, there may be additional interruption of 1 subframe due to the RRC reconfiguration procedure. While the switching interruption takes place for single RX UE (e.g. D2D discovery capable UEs), the RRC reconfiguration interruption takes place for all types of D2D UEs (e.g. D2D Discovery capable and D2D Communication capable).
As will be apparent from the following description, systems and methods that improve the above and other aspects of D2D operation are desired.